1. Field of the Invention
The methods and apparatus of the present invention relate generally to the field of heat dissipation for an integrated circuit (IC). More particularly, the present invention presents a method for providing high efficiency thermal interfaces in an IC package.
2. Art Background
Integrated circuits consume power during operation, generating heat. This heat must be carried away from the IC at a sufficient rate such that the temperature of the die remains within operating parameters. Both the operating temperature of the die and the number of watts dissipated are typically specified for each IC. For example, a typical die operating temperature is 95.degree. C. Failure to cool the IC properly can lead to the IC failing during operation.
A typical prior art solution uses a heat sink that is attached to the IC package in conjunction with an air-moving cooling system. A fan blows air onto the heat sink, transferring heat from the heat sink to the air. The fan causes the heated air to be carried away from the heat sink. The cooling requirements of the cooling system may be expressed by the following equation: EQU Q=.DELTA.T.multidot.C.sub.p .multidot.M (1)
Here, Q is the number of watts to be dissipated; .DELTA.T is the difference in temperature between the heat sink (T.sub.sink)and the ambient air (T.sub.air); C.sub.p is the thermal conductivity of air and is a constant; and M is the mass-flow of the air to be moved by the fan of the cooling system.
Typically, the interface between the heat sink and the package is of a low thermal efficiency. For example, a heat sink can be glued onto the outside of the package. The glue typically is of a higher thermal impedance than the heat sink. Further, there is some thermal impedance between the die and the package. The thermal impedances combine, preventing some heat from reaching the heat sink. The trapped heat or thermal overhead is a function of the number of watts dissipated by the IC and the thermal impedances between the die and the heat sink. This thermal overhead appears as a temperature gradient between the die and the heat sink. Thus, T.sub.sink is actually the temperature of the die (T.sub.die) minus the thermal overhead (T.sub.over). Every low efficiency thermal interface in the package/heat sink system adds to the thermal overhead.
In a low power device, the thermal overhead is not critical, but thermal overhead is of great concern in modern high-power ICs. It can be seen that if the thermal impedances remain the same for the high-power devices, T.sub.over increases. This is because, for design purposes, T.sub.die is a constant equal to the operating temperature of the IC (e.g. 95.degree. C.). Thus, T.sub.sink decreases, and the fan must provide a greater mass-flow of air in order to cool the IC. Practically speaking, the fan size most probably will be increased for very high-power devices, resulting in a corresponding increase in the noise signature of the computer system and greater cost. Such a solution is not desirable. Accordingly, the thermal overhead should be minimized to avoid the need for larger fans.
Therefore, as will be described, the method and apparatus of the present invention provide an improved integrated circuit package having minimal thermal overhead. More specifically, a heat pipe is integrally formed in the integrated circuit package, and a heat transfer assembly is fitted over the heat pipe using minimal pressure. The thermal interfaces thus provided are thermally efficient.